European Patent Application No. 51 092 discloses a picture display device in which the line deflection, i.e. the deflection in the horizontal direction of the electron beam(s) generated in the picture display tube, is symmetrical and, for example, sinusoidal. Compared with the asymmetrical, usually sawtooth-shaped line deflection, in which the (invisible) retrace is many times shorter than the trace, the symmetrical line deflection has a number of advantages, more specifically its lower power dissipation, the lower voltage load of the components in the line deflection circuit and the lower high-frequency radiation. These advantages are of particular importance when a high line frequency is used, for example higher than the frequency prescribed by a present-day television standard, the retrace period with the sawtooth-shaped deflection being very short, for example of the order of magnitude of 2 to 6 .mu.s. A description of the dissipation and of the voltage load at the sawtooth line deflection is given for different values of the line frequency in the article "Practical Considerations in the Design of Horizontal Deflection Systems for High-Definition Television Displays", published in the periodical "IEEE Transactions on Consumer Electronics", Vol. CE-29, No. 3, August 1983, pages 334 to 349.
In the arrangement disclosed in said European Patent Application said comparator stage is comprised of a sample-and-hold circuit to which the voltage present across the deflection coil is applied as a first signal. This voltage is measured during the occurrence of the signal from the counter, which signal occurs once in a period, and the measured value is maintained substantially constant by the operation of the control loop. In this way the video information is read at instants which are fixed relative to the line deflection. The position of the picture elements read can be set with the aid of a variable resistor. This setting means is included in the output lead of the sample-and-hold circuit and thus influences the control signal. The prior art arrangement also comprises a memory in which data are stored for supplying an additional control signal. Linearity errors are corrected with the aid of said means and picture elements of different picture lines which should be in the same horizontal position are indeed displayed along a vertical straight line.
The invention is based on the recognition that this object can only be accomplished if certain conditions are satisfied. If the amplitude of the voltage across the line deflection coil varies, then the value which is kept constant does not correspond with the appropriate picture element. This implies a horizontal shift of the video information into a certain direction for a line and into the opposite direction for the subsequent line. Also the setting effected with the aid of the variable resistor results in a voltage variation and consequently in a similar shift in the event of amplitude variations. Such a variation of the voltage across the deflection coil may happen as a result of spreads and/or temperature and ageing phenomena. In addition, for correcting the raster distortion the amplitude of the line deflection is modulated at the field frequency, so that said voltage across the coil varies during a field. So, if the picture elements are adjusted to the proper position for a given pair of lines, the position of corresponding picture elements above and/or below this pair is not the proper position in the displayed picture, even if errors due to spreads or the above-mentioned phenomena are absent, so that a vertical straight line to be displayed is not displayed as a vertical straight line. This results in an annoying frayed edge being displayed.